The invention concerns a process for the conversion of sewage sludge to humus as well as a polder for the carrying out of the process.
Large parts of the population have reservations concerning the use of sewage sludge as a fertilizer and for the improvement of the ground. Although legislation attempts to ensure the general public that there are ordinances for indirect polluters of public sewage sludge, parts of the public fear an accumulation of harmful substances in the ground.
Furthermore, the surplus sludge of the waste water treatment plant is of very liquid consistency (99% water). In order to keep the transportation costs within limits it is necessary to carry out expensive dehydration processes.
One known process for the conversion of sludge to humus applies reeds (Phragmatis). These plants, however, are very expensive as they must be cultivated in bales. Furthermore, the strong stalks and roots of the reed require increased working before the converted sludge can be processed. Due to the regular filling with sludge in the known process the degree of complete drying of the converted sludge is relatively low. As the reed plants are used over the entire filling period of 10-15 years the percentage of decomposed plants is relatively low.
German Patent No. DE 43 23 610 describes a bio-polder (interim storage) for the biological clarification and/or mineralization of soil and sludge and for the biological clarification of waste water that is polluted by organic and inorganic substances. The bio-polder usually contains at least one aerobic and one anaerobic layer that lie on top of each other. The harmful substances that have not been eliminated in the aerobic layer are drawn into the anaerobic layer by gravity and are subjected to successive oxidative and reducing processes.
The invention is the result of the task to find a process for the conversion of sewage sludge to humus avoiding chemicals. This task is met for the process for the conversion of sludge to humus by the characteristics as stated in the describing part of claim 1.
Further development and advantageous arrangement of the invention arise from the further claims, the description, and the drawings.